The project addresses neural mechanisms whereby olfactory systems code stimulus quality. This will be accomplished by examining the relationship between dimensions of neural response and dimensions of sensory experience for a set of defined odorants, employing analytical techniques developed to study gustatory quality coding but never before applied to olfaction. The project uses a model, the spiny lobster, exploiting the fact that this organism has a well-developed, glomerular-type olfactory system and that neural activity and feeding behavior in this organism are elicited by mixtures of simple, easily identified and quantified organic molecules. One aim is to analyze how this animal behaviorally classifies odorant molecules, using aversive conditioning procedures. This information is important because, even though many of the most effective, behaviorally relevant odorants for this animal have been identified, it is not known what constitutes a single odor quality for this animal (i.e., which odorants are perceived as smelling similar). Then, toward delineating the neural code for these odor qualities, electrophysiological methodology will be used to study the response properties of single olfactory interneurons in a major olfactory processing center of the brain. These interneurons are third-order neurons in the olfactory pathway. The responses and coding mechanisms of neurons at this level can then be compared to previously described responses of neurons at the first two levels of the olfactory pathway. This will serve to determine whether similarities in the odorant-elicited patterns of neural activity across all cells at a given neural level (i.e., across-neuron patterns) can account for similarities in the perceived smells of the odorants, and to evaluate the fate and function of receptor cells and low-order interneurons with narrowly-tuned response spectra. Additionally, morphological and physiological properties of single interneurons will be correlated in order to determine the structural basis for the functional organization of this olfactory system.